1. Field of the Invention
This invention relates to semiconductor devices and methods of manufacture. More particularly, the invention relates to thin film diodes having large current capability and methods of manufacture of such diodes on arbitrary substrates.
2. Description of Related Art
Silicon-based diodes, whether in discrete form or integrated on a circuit fulfill most application needs, for a large range of power requirements. There are, however, applications where it is desirable to fabricate diodes on arbitrary substrates. Examples include switching elements for active matrix liquid crystal displays or solar cells.
A popular material for these applications has been amorphous hydrogenated silicon (a-Si:H), typically deposited by plasma chemical vapor deposition processes (CVD). The main advantages are the compatibility with large substrates and low temperature processing and most importantly, the ability of doping during the deposition process. A large variety of a-Si:H device structures have been demonstrated: p/n, p/i/n and n/i/n junction, and Schottky diodes, thin film diodes (TFDs) and thin film transistors (TFT).
A limitation of amorphous hydrogenated silicon is its large resistivity (i.e., low mobility) which ranges from 10.sup.8 ohm-cm to roughly 10.sup.3 ohm-cm for an undoped and a heavily doped material, respectively. As a consequence, the forward characteristics of amorphous hydrogenated silicon above one volt are dominated by bulk resistance, limiting the current density at this voltage to about 1-10 Amperes per cm.sup.2 (A/cm.sup.2). Larger current densities could be achieved with the use of polycrystalline silicon, however, at significant costs in process complexity.
Large current capability in thin film diodes (TFD's), even outside the amorphous hydrogenated work, has not been generally emphasized, yet it could find important applications. Consider for example, the problem of providing electrostatic discharge (ESD) protection to microelectronic devices. On single crystal semiconductor wafers, ESD protect devices can be easily integrated with no additional complexity. In this context, we refer to "integrated" to signify a device fabricated on the same wafer surface, excluding therefore, hybrid arrangements incorporating discrete components. In the absence of a single crystal semiconductor substrate, an integrated ESD protect circuit requires the use of TFD's or TFT's.
In one embodiment of the present invention, TFD's have been used for wafer-level ESD protection of magnetoresistive (MR) sensors. The MR sensor is a thin metal film resistor (typically a few tens of ohms in resistance) of extremely small volume. The very limited ability to dissipate heat makes the MR sensor vulnerable to ESD current transients. A TFD connected in parallel could alleviate the problem by shunting the ESD current pulse. Consider for simplicity only one type of ESD discharge, the so-called Human Body Model type discharge, in which a 150 pF capacitor is charged to a voltage and discharged through the MR sensor with a 1500 ohm series resistor. When the capacitor is charged to 1500 volts, the discharge produces a current pulse of 1 Ampere (A) peak current with a 150 nanoseconds (ns) decay time constant. To be effective in protecting against these ESD events, while not degrading sensor performance, a shunting TFD must fulfill the following requirements:
(a) Turn on voltage not below the operating voltage of the sensor (0.3 to 0.5 volts) but not too high (&lt;/=1 volt); PA1 (b) Low dynamic on resistance (1 to 2 ohm); PA1 (c) Ability to survive discharges of 1 A peak current (combined with an area constraint of 4.times.10.sup.-4 cm.sup.2 corresponds to peak current densities of 2,500 A/cm.sup.2); and PA1 (d) Diode capacitance &lt;20 pF. PA1 (a) Simple p-n junctions are ohmic, not rectifying; diode behavior is only achieved in p/i/n structures with highly resistive layers at the active junction; PA1 (b) The turn-on voltage is of the order of several volts, that is roughly 10 times greater than required for TFD's to be effective as an ESD shunt; and PA1 (c) Large current capability has not been demonstrated.
Nickel oxide (NiO.sub.x) and Indium Tin Oxide (ITO) have been found to be suitable materials for use in a TFD which would meet the above requirements. The NiO.sub.x provides a p-type semiconductor layer while the ITO provides an n-type semiconductor layer to form a p/n junction diode.
In the prior art, one example of a thin film diode based on a junction between nickel oxide and an n-type conductive oxide is reported in "Transparent Conducting P-Type NiO.sub.x Thin Films Prepared by Magnetron Sputtering" by H. Sato et al., published in the Thin Solid Film Journal, Volume 236, pages 27-31, 1993. Sato et al., uses an n-type conductive oxide, zinc oxide, as an n-type layer. The Sato et al. diodes differ in several essential ways from the diodes described above, as follows:
Indium tin oxide layers have also been used in metal diodes to provide or control the driving voltage for signal conductors in a liquid crystal display as shown in U.S. Pat. No. 5,253,092 entitled "Lateral MIM Device and Method of Production" issued Oct. 12, 1993 to K. Takahashi. The described indium tin oxide layer is used in combination with chromium (Cr) as a conductive layer. Zinc oxide is used as an insulating layer. The combination of NiOx and indium tin oxide as conductive layers in a thin film diode is not shown or disclosed in the Takahashi patent.
U.S. Pat. No. 5,272,370 entitled "Thin-Film ROM Devices and Their Manufacture" issued Dec. 21, 1993 to I. French, describes another utilization of indium tin oxide in the fabrication of metal insulator metal diodes. Again, NiO.sub.x is not shown or suggested in the French patent.
None of the prior art demonstrates a p/n junction thin film diode using NiO.sub.x and ITO which can be deposited on arbitrary substrates at low temperature using conventional deposition techniques. Nor does the prior art demonstrate applications having low turn-on voltage (0.5 volts) and large forward current capability (DC current densities greater than 10,000 A/cm.sup.2). Moreover, the prior art does not provide low dynamic on resistance which makes available electrostatic discharge protection. The present invention solves the problem of providing TFD's having high current capability with low turn-on voltage, low dynamic resistance and low capacitance.